Polymeric aromatic carbamic acid esters (polyurethanes) such as diphenylmethane dicarbamates and the related higher homologs polymethylene polyphenyl carbamates have become increasingly important products, particularly for use in the preparation of the commercially valuable diphenylmethane diisocyanates and mixtures of diisocyanates and polyisocyanates by the decomposition of such polymeric aromatic carbamic acid esters in a suitable solvent as shown, for example, in Rosenthal et al, U.S. Pat. Nos. 3,962,302 and 3,919,279.
At the present time there is no known successful commercial method for the direct prepartion of polymeric aromatic esters of carbamic acid. The corresponding diphenylmethane diisocyanates and polyisocyanates, available commercially, are largely produced by the phosgenation of mixtures of diamines and polyamines obtained by the condensation of aniline and formaldehyde with catalytic quantities of a mineral acid, as for example, disclosed in the Pistor et al, U.S. Pat. No. 4,041,914.
Prior art processes have been proposed for the preparation of polymeric aromatic carbamic acid esters (polyurethanes) as for example in Klauke et al, U.S. Pat. No. 2,946,768 and British Pat. No. 461,352 which disclosed the condensation of aryl carbamic acid esters with carbonyl compounds such as aldehydes and ketones in a dilute aqueous mineral acid condensation medium. In such processes the carbonyl compound such as formaldehyde tends to react at the nitrogen of the carbamate to produce along with the desired polyurethanes, substantial amounts, i.e., generally between 15 percent and 50 percent by weight, of undesirable N-(alkoxycarbonyl)phenylaminomethylphenyl compounds which includes dimers, trimers, tetramers, etc. of such compounds, which compounds referred to as "N-benzyl" compounds are fully described in co-pending U.S. application, Ser. No. 905,705, filed May 15, 1978, now U.S. Pat. No. 4,146,727. Attempts to prepare diisocyanates and polyisocyanates or to otherwise use the mixture containing the polyurethanes and such amounts of the undesired compounds, which compounds cannot be converted to an isocyanate by pyrolysis, presents many problems since there is no known method for separating the polyurethanes from the N-(alkoxycarbonyl)phenylaminomethylphenyl impurities.
In addition, the liquid acid catalysts such as sulfuric or a sulfonic acid must be separated from the reaction mixture, usually by extensive water washing and the recovered acid reconcentrated for the normally desirable recycle and reuse.
The present invention which comprises the preparation of diphenylmethane dicarbamates and polymethylene polyphenylcarbamate homologs and derivatives of these compounds by the condensation of N-aryl carbamic acid esters with formaldehyde, para-formaldehyde or trioxane in the presence of a graphite supported Lewis acid catalyst such as aluminum trichloride, iron (III) chloride and antimony pentafluoride yields a product relatively low in N-benzyl content while at the same time it improves the process by using an acid catalyst which is substantially insoluble in the reaction medium and thus eliminates problems of acid recovery. The catalyst may be recovered from the reaction mixture by simple filtration and dried.
While the process of the invention is a one step condensation process, it is often convenient to further treat the condensation reaction product with additional Lewis acid or other solid acid catalyst, after removal of water made during the condensation reaction, to convert or rearrange any contained "N-benzyl" compounds to the desirable carbamates as described in the above mentioned U.S. application, Ser. No. 905,705, filed May 15, l978, now U.S. Pat. No. 4,146,727, and the catalyst again recovered simply by filtration.
The graphite supported Lewis acid catalysts (intercalation compounds) of the instant invention also substantially eliminate corrosion problems and the undesirable side reactions such as sulfonation of the N-aryl carbamates and chlorination which occur for example with sulfuric and hydrochloric acid. Since the side reactions are suppressed, a higher quality diphenylmethane dicarbamate and polymethylene polyphenyl carbamate product can be obtained which upon pyrolysis to the polymeric aromatic isocyanate gives a higher isocyanate yield.